1. Field of the Invention
This invention relates to a semiconductor fabrication process, and more particularly, to a method for forming a borderless contact hole.
2. Description of Related Art
In order to achieve higher integration of a semiconductor device, a borderless contact process has been implemented in the most current semiconductor fabrication processes. Even though the borderless contact process is able to efficiently downsize a semiconductor device to increase the integration, problems, such as how to effectively control the etching process with a properly selected etching stop layer and etchant, still need to be resolved.
A currently used borderless contact process is schematically illustrated in FIGS. 1A through 1C.
Referring to FIG. 1A, a silicon nitride layer 14 is formed on a substrate 10 to cover preformed devices including an isolation structure 11, a gate 12 and source/drain regions 13 of a metal-oxide-semiconductor (MOS) transistor. The silicon nitride layer 14 serves as an etching end point, and provides protection to the isolation structure 11 in the follow-up etching process. A silicon oxide layer 15 is then formed on the silicon nitride layer 14.
Referring next to FIG. 1B, an etching process 17 is performed on the silicon oxide layer 15 to form an opening 16 that exposes a portion of the silicon nitride layer 14, which is the etching stop layer. Normally, the etching process 17 is performed on a magnetically enhanced reactive ion etching (MERIE) etcher along with gaseous C.sub.4 F.sub.8 /CO/O.sub.2 /Ar as an etching gas. Then, by removing the exposed silicon nitride layer 14 within the opening 16, a contact hole 16a that exposes a portion of the source/drain regions 13 is finished, as shown in FIG. 1C.
Because the oxide-to-nitride selectivity is the most critical parameter for the foregoing etching process, the profile of the contact hole 16a directly relates to the oxide-to-nitride selectivity of the selected etchant. Once an etchant with an improper oxide-to-nitride is used in the foregoing process, a problem, either over-etching the silicon nitride layer 14 as shown in FIG. 2A, or an abnormal termination of the etching process as shown in FIG. 2B, further causes electrical malfunction. While an etching process is performed on the silicon oxide layer 15 to form an opening 17, the etching process is terminated as soon as the silicon nitride 14 is exposed. However, with an improperly selected etchant, a defective opening 17, as shown in either FIGS. 2A or 2B, is obtained. A tapered profile opening 19 as shown in FIG. 2C is also a possible defective opening caused by a improperly selected etchant.
As shown in FIG. 2D, because polymer molecules generated by the etchant normally tend to deposit on the silicon oxide near the corners of the opening 20 that further abnormally affects the isotropy of the performed etching process, a bowed profile opening 20 is obtained. Since in the fabrication process of a highly integrated semiconductor device, a vertical profile, which has lateral walls forming angles of nearly 90 degrees to the surface of the substrate, is required to meet the desired design rule, a bowed profile opening 20 is definitely unable to meet the requirement.
An etching process that uses an etchant with a low oxide-to-nitride selectivity normally leads to etch through the etch stop layer as shown in FIGS. 2A. On the other hand, an etchant with a high oxide-to-nitride selectivity tends to cause an under-etched opening, a tapered profile and/or a bowed profile opening shown in FIGS. 2B, 2C and 2D. Therefore, conventionally, a borderless contact process can only perform on either one of the options, a vertical profile or a better control of the etching end point with high oxide to nitride selectivity, accordingly to the actual needs.